This invention relates to Gifford-McMahon, hereinafter referred to as G-M, refrigerator cold heads of the type that have sub-displacer assemblies which are dynamically balanced. Such structures of this type generally eliminate substantially all of the noise and vibration in the cold head by reciprocating the sub-displacers in opposing sinusoidal motions so that the inertias of the sub-displacers due to the opposing, sinusoidal motion should cancel each other out. In particular, a G-M cold head is constructed of at least two sub-displacers, having springs located on both ends of the sub-displacers, such that the sub-displacers are sinusoidally driven by a pressure drive and the sinusoidal movement of the sub-displacers should cause the inertia of the sub-displacers created by the sinusoidal movement to be cancelled which, in turn, should substantially eliminate noise and vibration in the cold head. This invention relates to certain unique G-M cold head assemblies and the noise and vibration reduction means in associations therewith.
Typically, the G-M cold head is used in magnetic resonant imaging devices. Any type of extraneous vibration or noise adversely affects the quality of the images produced by the devices such that "ghosts" or "artifacts" can be created in the images if undue vibrations are experienced in the device.
In view of this, it is known, in prior G-M cold head systems, to make use of a cold head system including a displacer drive system to cool the coils in an imaging device. In each of these cases, the inertia of the displacer was not controlled which resulted in "displacer banging" which, in turn, could create ghosts or artifacts in the images.
Exemplary of such prior art G-M cold head systems are the undirectional mechanical mechanism and the pneumatic displacer drive mechanism. The unidirectional mechanical mechanism, typically, employs a conventional Scotch-yoke mechanical drive mechanism to drive the first and second stage displacers. This well-known Scotch-yoke drive mechanism is inherently noisy and could create undue vibrations which may adversely affect the quality of the images produced. Also, the individual mechanical mechanism employs a second stage low temperature seal which should separate the gases between the first and second stage displacers, which are maintained at temperatures of 50 K. in the first stage and 10 K. in the second stage. However, the seal, typically, wears out after a relatively short period of time and the gases, at those different temperatures, could begin to mix which may adversely affect the mechanism by creating an undue heat load upon the mechanical mechanism.
With respect to the pneumatic displacer drive mechanism, typically, the drive mechanism has at least three separate cavities having a particular volume associated with that particular cavity. The displacer is driven by a pressure/force imbalance created between the cavities. While this device is relatively simple in its construction, there is relatively no means provided for dampening the displacer, thus displacer banging could be relatively acute in this device. In view of the prior art as set forth above, a more advantageous system, then, would be presented is such amounts of noise, vibration and seals were reduced.
It is apparent from the above that there exists a need in the art for a G-M cold head system which adequately cools the coils in the superconductive magnet of an imaging device, and which at least equals the cooling efficiency of known G-M cold heads, but which at the same time substantially reduces the noise and vibration associated with displacer banging or the like. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.